Topics Covered

Background

Grades 321 and 347 are the basic austenitic 18/8 steel (Grade 304) stabilised by Titanium (321) or Niobium (347) additions. These grades are used because they are not sensitive to intergranular corrosion after heating within the carbide precipitation range of 425-850°C. Grade 321 is the grade of choice for applications in the temperature range of up to about 900°C, combining high strength, resistance to scaling and phase stability with resistance to subsequent aqueous corrosion.

Grade 321H is a modification of 321 with a higher carbon content, to provide improved high temperature strength.

A limitation with 321 is that titanium does not transfer well across a high temperature arc, so is not recommended as a welding consumable. In this case grade 347 is preferred - the niobium performs the same carbide stabilisation task but can be transferred across a welding arc. Grade 347 is therefore the standard consumable for welding 321. Grade 347 is only occasionally used as parent plate material.

Like other austenitic grades, 321 and 347 have excellent forming and welding characteristics, are readily brake or roll formed and have outstanding welding characteristics. Post-weld annealing is not required. They also have excellent toughness, even down to cryogenic temperatures. Grade 321 does not polish well, so is not recommended for decorative applications.

Grade 304L is more readily available in most product forms, and so is generally used in preference to 321 if the requirement is simply for resistance to intergranular corrosion after welding. However 304L has lower hot strength than 321 and so is not the best choice if the requirement is resistance to an operating environment over about 500°C.

Key Properties

These properties are specified for flat rolled product (plate, sheet and coil) in ASTM A240/A240M. Similar but not necessarily identical properties are specified for other products such as pipe and bar in their respective specifications.

Composition

Typical compositional ranges for grade 321 stainless steels are given in table 1.

Table 1. Composition ranges for 321 grade stainless steel

Grade

C

Mn

Si

P

S

Cr

Mo

Ni

N

Other

321

min.

max

-

0.08

2.00

0.75

0.045

0.030

17.0

19.0

-

9.0

12.0

0.10

Ti=5(C+N)

0.70

321H

min.

max

0.04

0.10

2.00

0.75

0.045

0.030

17.0

19.0

-

9.0

12.0

-

Ti=4(C+N)

0.70

347

min.

max

0.08

2.00

0.75

0.045

0.030

17.0

19.0

-

9.0

13.0

-

Nb=10(C+N)

1.0

Mechanical Properties

Typical mechanical properties for grade 321 stainless steels are given in table 2.

Grade Specification Comparison

Approximate grade comparisons for 321 stainless steels are given in table 4.

Table 4. Grade specifications for 321 grade stainless steel

Grade

UNS No

Old British

Euronorm

Swedish SS

Japanese JIS

BS

En

No

Name

321

S32100

321S31

58B, 58C

1.4541

X6CrNiTi18-10

2337

SUS 321

321H

S32109

321S51

-

1.4878

X10CrNiTi18-10

-

SUS 321H

347

S34700

347S31

58G

1.4550

X6CrNiNb18-10

2338

SUS 347

These comparisons are approximate only. The list is intended as a comparison of functionally similar materials not as a schedule of contractual equivalents. If exact equivalents are needed original specifications must be consulted.

Possible Alternative Grades

Possible alternative grades to grade 321 stainless steels are given in table 5.

Table 5. Possible alternative grades to 321 grade stainless steel

Grade

Why it might be chosen instead of 321

304L

The requirement is for resistance to intergranular corrosion, not for high temperature strength

3CR12

Only mild "high temperature" environment is present… up to about 600°C.

304H

Only mild "high temperature" environment is present… up to about 800°C.

310

The temperature of the operating environment is up to about 1100°C - too high for 321 or 321H.

S30815

(253MA)

The temperature of the operating environment is up to about 1150°C- too high for 321 or 321H.

Corrosion Resistance

Equivalent to Grade 304 in the annealed condition, and superior if a weldment in these grades has not been post-weld annealed or if the application involves service in the 425-900°C range. Subject to pitting and crevice corrosion in warm chloride environments, and to stress corrosion cracking above about 60°C. Considered resistant to potable water with up to about 200mg/L chlorides at ambient temperatures, reducing to about 150mg/L at 60°C.

Heat Resistance

Good oxidation resistance in intermittent service to 900°C and in continuous service to 925°C. These grades perform well in the 425-900°C range, and particularly where subsequent aqueous corrosive conditions are present. 321H has higher hot strength, and is particularly suitable for high temperature structural applications.

Heat Treatment

Stabilising - heat to 870-900°C for 1 hour per 25mm of thickness and air cool. Stabilisation is recommended for most severe service conditions (above 425°C) and particularly for material annealed at the upper side of the annealing temperature range.

Stress Relief - Heat to 700°C for 1 to 2 hours and air cool.

These grades cannot be hardened by thermal treatment.

Welding

Excellent weldability by all standard fusion methods, both with and without filler metals. AS 1554.6 pre-qualifies welding of 321 and 347 with Grade 347 rods or electrodes; high silicon version of 347 is also pre-qualified for welding of 321.